Many diseases can increase the fat or iron deposition of tissues compared to normal status. For example, with respect to liver diseases, hepatic steatosis is an indication of hepatic diseases such as nonalcoholic fatty liver disease. Additionally, increased iron deposition is associated with chronic viral hepatitis, alcoholic liver disease, and nonalcoholic steatohepatitis. Outside of the area of liver diseases, the quantification of fat and iron deposition is of interest in areas such as the diagnosis of bone marrow diseases, the characterization of adrenal masses, and the evaluation of heart diseases. Therefore, the rapid and accurate evaluation of fat or iron deposition in anatomy is important in a variety of clinical applications.
One technique for non-invasively diagnosing and quantifying fat deposition in tissues is Magnetic Resonance Imaging (MRI). In MRI, the observed signal of a normal tissue is primarily from the protons in its constituent water molecules. However, in tissues with high levels of fat, the observed signal is a combination of fat and water signals. The total signal corresponding to an image depends on the number of protons, or proton density (PD). To quantify fat in the tissue, the ratio of fat proton density to total fat and water proton density per pixel may be determined. This ratio is known as the proton density fat fraction (PDFF). Various conventional techniques exist for calculating PDFF. However, these methods are typically tied to, and thus limited to, specific vendor/hardware platforms.
Measuring iron with MRI is usually accomplished by measuring the tissue transverse relaxation values (T2 or T2*) or relaxation rates (R2=1/T2 or R2*=1/T2*) with MRI, because iron deposition is associated closely with T2/T2* or R2/R2*. This is generally accomplished by acquiring multi-echo data and performing a log-linear fitting. However, in the presence of fat, directly measuring T2/T2* or R2/R2* by this kind of method is problematic due to the influence of fat.
Thus, it is desired to develop a technique that allows PDFF calculation across various vendor-specific platforms, while also allowing iron deposition to be accurately quantified.